1. Field of Art
This invention relates to relay control circuitry, and more particularly to accurate, fail-safe relay timer circuitry.
2. Description of the Prior Art
It is known that there are many applications where accurate timing of relay operation is required. For instance, in elevator control systems, there are numerous functions controlled by relays which must have relatively accurate timing. In elevator systems, the circuitry must also be fail-safe so as to avoid breakdown of elevator service, or the trapping of or injury to passengers.
Typical relay timing circuitry involves a charged capacitor, the relay resistance, capacitance and relay dropout voltage determining the timing of relay dropout. Additional variable resistance in the RC time constant path, such as a potentiometer, may be utilized to adjust the timing of such a circuit, to overcome variations in relay coil resistance. However, such circuits rely on the decay of voltage below relay dropout voltage to cause disenergizing of the relay; this in turn renders the timing of the circuit dependent upon the particular relay dropout voltage, which may vary widely from one relay to the next. Additionally, since the initial charging voltage is a function of the operating voltage, the time necessary to decay to the relay dropout voltage can vary as a function of operating voltage variations.
Other relay timer circuits may employ a transistor in series with the relay coil. This can provide accurate and rapid turnoff of the relay power, but failure of the transistor in a shorted or open condition forces the relay to be always on or always off, respectively. Since such circuitry frequently has to be in a somewhat hostile environment (such as subject to high acceleration and vibration) the failure incidence is higher than that which can be tolerated in many circumstances. And frequently, failure of such a relay can be hazardous, dependent upon the function which it performs.